When electric current flows through a circuit, it encounters inherent electrical resistance or impedance as it passes through respective elements in the circuit. As established by Ohm's Law, the result of current moving through a circuit element is a voltage drop across that element. Often a circuit element can withstand only a maximum voltage drop, and may be damagedby an excessive or overvoltage condition.
Power systems that supply current or voltage to a circuit can be provided with overvoltage detectors. Such detectors can be used to sense voltage drops across circuit elements, or loads, that are at risk of encountering overvoltage conditions, but most look at voltage across terminals that are being voltage-regulated or current-regulated by the supply. Some power systems can regulate the voltage across load terminals, instead of power supply terminals, as away to achieve a more precise load voltage regulation. In order for load voltage regulation to be effective in these systems, voltage sense leads must be correctly connected to the appropriate load terminals before the load is energized. The overvoltage detector can then monitor voltage across the load terminals, preferably through the same sense leads, and if an overvoltage condition is sensed, i.e., a voltage that is greater than the threshold value, then the power system is shut off in order to prevent damaging the load. Unfortunately, if the load voltage sensors are not connected properly, for example if they are shorted together, an overvoltage condition can occur without the power system sensing it, resulting in damage to the load. Without supplemental protection, improperly connected sense leads will likely result in the load's voltage exceeding its regulation limits, and perhaps exceeding a maximum safe limit if that is within the compliance range of the power system, when it is energized. This is due to the normal behavior of gain limited negative feedback regulation employed by state of the art power systems that would act to produce greater supply terminal voltage in an attempt to increase the voltage across its shorted sense connections.
Some prior art systems with load voltage sensing capability provide supplemental overvoltage detection at the supply terminals in addition to basic detection at the load terminals. These schemes require a second voltage threshold that allows for load voltage, voltage drop along the distribution cables, component value tolerances, circuit noise and offset voltages. In some systems, the supplemental overvoltage threshold is programmable by an operator for greater precision, and in others may be fixed in hardware as determined by the system designer, providing a less precise supplemental protection.
Known prior art supplemental overvoltage protection schemes where thresholds are fixed in hardware can be incapable of providing load overvoltage protection when the load overvoltage limit is below the supplemental threshold. This condition can exist for circuits where the maximum expected cable voltage drop is significant relative to that of the load. Setting a threshold with hardware that allows both for maximum cable voltage drop and load voltage drop will also allow a supply terminal voltage to be present that could damage the load when a lesser actual cable voltage drop is present. One extreme case would exist for an unlikely condition of zero actual cable impedance. More realistic cable impedance would most likely fall between some minimum value and a maximum specified value. In this modality for ineffecfive supplemental protection in prior schemes, low cable impedance interferes with the supplemental circuit's effectiveness in protecting the load, and subsequently protection does not work if the load overvoltage limit is below the supplemental threshold less the actual total cable voltage drop. The length of the cable, size of the cable's conductor cross-section, type of conductor, temperature of the conductor, cable construction, and cable routing all contribute to determining the actual impedance of distribution cables intended for most system installations. Other modalities for failure of known prior art non-programmable protection schemes may also exist that are not described here.
For some loads, greater absolute precision may be required in enforcing a supplemental overvoltage threshold, directing design of those prior art supplemental schemes preferably toward requiring programming of the supplemental threshold value rather than having it fixed in hardware. Human programming error, if it occurs, would likely render the more precise supplemental overvoltage detection system ineffective.
Causes for failure of overvoltage load protection therefore can include human programming error, human connection error or hardware failure involving sense lead connections, and inability of known protection schemes to protect low voltage loads with possibly significant expected cable voltage drops between zero and the maximum safe load voltage.